Magnesium die cast components, because of their weight advantage and other characteristics, are used in automobiles and other mobile equipment. Obviously there are numerous other uses for die cast magnesium parts and pieces i.e. computer housings and chain saw housing only to mention a few.
One of the major drawbacks with known magnesium die casting is extensive wastage because of flaws that result when using existing processes and equipment.
Temperatures and minimum temperature variations are critical as well as other process variable parameters such as metal level in the molten bath and consistent metal pour to meet the unique properties of a magnesium melt operation.
Some considerations for making a quality magnesium casting are as follows:
(1) The ingot (15 lbs. or 25 lbs.) should be suitably conditioned (i.e. pre-heated) before being placed into the furnace metal bath. For example should moisture be left in or on the ingot that moisture will become super heated steam in the furnace which can cause an explosion to occur in the metal bath.
One method of preheating used in the prior art is to lay the ingots on top of the furnace and when the operator felt they were ready then place them by hand in the molten bath. Another method, when using gas fired furnace, was to use the heat radiated from the furnace, as well as ducting arrangement of the vented heat to be channelled into an enclosure where this heat was used to preheat the ingots. A further method was using an electric duct heater in a preheat chamber with little or no heat control and with no regard to or determination or measurement of the ingot temperature. All of the above methods are extremely dangerous as they do not meet the various ingot supplier warning of never introducing a magnesium ingot into the metal bath until it is above 150.degree. C. (300.degree. F.). An ingot below the noted temperature also creates temperature gradients in the metal bath which produce dross and sludge. There also is the risk of explosion. The end result is scrap production parts, down time and possible injury to personnel and equipment.
(2) Temperature variations are critical. For example the metal bath (furnace) should stay within a selected temperature range when an ingot has been charged. The metal bath should not deviate above or below a preselected temperature. If it does dross build up can occur which filters down through the metal bath allowing impurities to become part of the casting. Castings with imperfections become costly scrap.
(3) The metal bath level should remain at as constant a level as possible to ensure each metal pour is the same amount, and to be able to realize the same amount of head pressure for each pour. In one prior art method the machine operator would, after making a certain number of parts, go to the furnace and add ingots to bring the metal level to where he wanted it. In another method after making a certain number of parts, a counter in the die cast machine would turn on a lamp to indicate ingots needed to be added to the metal bath. The operator would then add ingots by hand to bring the level to the desired level. These methods rely totally on the operator who may or may not be conscientious to the need of a constant metal level for operating in process control. The operator is also at risk, if the ingots being added are below a safe temperature to do so.
(4) Transfer of the metal from the furnace through the siphon tube should be done as quickly as possible due to the rapid loss of heat from the magnesium when contact with the atmosphere as well as the burning that occurs, which contaminates the metal pour.
In the prior art melting of magnesium one method used is gas fired furnaces, normally with two combustion blower units. This is an effective method for melting the magnesium but by its characteristics alone makes an extremely poor choice for magnesium due to the following:
(a) magnesium dust is extremely flammable and should never be in an environment with an open flame; and
(b) due to the extremely large swing in the temperature above and below the set point (typically .+-.25.degree. C.), there is no chance to keep the metal bath temperature within the extremely tight metal bath requirements. The metal bath temperature desirably should be .+-.8.degree. C., in relation to the metal bath set point.
Another method used is an electric furnace with power contactor relays where the amount of power is selected by a selector switch and the overall metal bath controller is used to control the on/off operation. This furnace is in actual fact a holding furnace used to maintain temperature. It would normally be filled with liquid metal from a smelter. Also, since it is not interactive to a preheater control structure, or a die cast machine, it has no ability to anticipate the ingot being introduced to the metal bath or to what the die cast machine mode of operation is.
The following are a few of the prior art systems of transferring liquid magnesium from a furnace metal bath to a die cast machine shot sleeve:
(1) Operator uses hand held ladle to ladle magnesium to the die cast machine pour hole. This method is time consuming and may only be used for small parts, i.e. one to two pounds. The operator must also ensure sulphur powder is constantly introduced to the metal bath surface to prevent burning of the liquid magnesium that is in contact with the atmosphere;
(2) Another method is the use of mechanical pumps which are operated by air and function as a piston style pump device. These units are prone to breakdown due to the high metal temperatures and are also prone to metal freeze up in the delivery pipe and pump itself if not kept in constant operation. Should there be metal freeze up the pump must be pulled out of operation and flushed out with sulphuric acid to clear obstructions. This method and the above other methods are very poor in relation to a constant quality of the desired amount of liquid magnesium being sent repeatedly time after time to the shot sleeve pour hole;
(3) Another method is the use of inert gas displacement and pressure transfer. This method uses inert gas to pressurize a crucible area. The inert gas tube goes into the crucible area and the delivery tube to the shot sleeve leaves and goes to the shot sleeve pour hole. This is a costly method as it involves electrically heating the transfer tube as well as the cost of the inert gas and pressurizing. This method also is inadequate in relation to process control due to the amount of liquid magnesium introduced to the shot sleeve. It varies greatly from one pour to the next.
Both items (2) and (3) introduce extra ancillary equipment that are unnecessary.
The foregoing are a few of the problems consistent with die casting magnesium but the major problem is the fact that a totally integrated process has never been developed before now to include all the process requirements from raw ingot to pour of the metal into the shot sleeve hole.